Add the __NR_bpf definitions to fix the following build errors for mips.

  $ cd tools/bpf/bpftool
  $ make
  [...]
  bpf.c:54:4: error: #error __NR_bpf not defined. libbpf does not support your arch.
   #  error __NR_bpf not defined. libbpf does not support your arch.
      ^~~~~
  bpf.c: In function ‘sys_bpf’:
  bpf.c:66:17: error: ‘__NR_bpf’ undeclared (first use in this function); did you mean ‘__NR_brk’?
    return syscall(__NR_bpf, cmd, attr, size);
                   ^~~~~~~~
                   __NR_brk
  [...]
  In file included from gen_loader.c:15:0:
  skel_internal.h: In function ‘skel_sys_bpf’:
  skel_internal.h:53:17: error: ‘__NR_bpf’ undeclared (first use in this function); did you mean ‘__NR_brk’?
    return syscall(__NR_bpf, cmd, attr, size);
                   ^~~~~~~~
                   __NR_brk

We can see the following generated definitions:

  $ grep -r "#define __NR_bpf" arch/mips
  arch/mips/include/generated/uapi/asm/unistd_o32.h:#define __NR_bpf (__NR_Linux + 355)
  arch/mips/include/generated/uapi/asm/unistd_n64.h:#define __NR_bpf (__NR_Linux + 315)
  arch/mips/include/generated/uapi/asm/unistd_n32.h:#define __NR_bpf (__NR_Linux + 319)

The __NR_Linux is defined in arch/mips/include/uapi/asm/unistd.h:

  $ grep -r "#define __NR_Linux" arch/mips
  arch/mips/include/uapi/asm/unistd.h:#define __NR_Linux	4000
  arch/mips/include/uapi/asm/unistd.h:#define __NR_Linux	5000
  arch/mips/include/uapi/asm/unistd.h:#define __NR_Linux	6000

That is to say, __NR_bpf is
4000 + 355 = 4355 for mips o32,
6000 + 319 = 6319 for mips n32,
5000 + 315 = 5315 for mips n64.

So use the GCC pre-defined macro _ABIO32, _ABIN32 and _ABI64 [1] to define
the corresponding __NR_bpf.

This patch is similar with commit bad1926dd2f6 ("bpf, s390: fix build for
libbpf and selftest suite").

[1] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/mips/mips.h#l549

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---

v2: use a final number without __NR_Linux to define __NR_bpf
     suggested by Andrii Nakryiko, thank you.

  tools/build/feature/test-bpf.c |  6 ++++++
  tools/lib/bpf/bpf.c            |  6 ++++++
  tools/lib/bpf/skel_internal.h  | 10 ++++++++++
  3 files changed, 22 insertions(+)

diff --git a/tools/build/feature/test-bpf.c b/tools/build/feature/test-bpf.c
index 82070ea..727d22e 100644
--- a/tools/build/feature/test-bpf.c
+++ b/tools/build/feature/test-bpf.c

@@ -14,6 +14,12 @@
  #  define __NR_bpf 349
  # elif defined(__s390__)
  #  define __NR_bpf 351
+# elif defined(__mips__) && defined(_ABIO32)
+#  define __NR_bpf 4355
+# elif defined(__mips__) && defined(_ABIN32)
+#  define __NR_bpf 6319
+# elif defined(__mips__) && defined(_ABI64)
+#  define __NR_bpf 5315
  # else
  #  error __NR_bpf not defined. libbpf does not support your arch.
  # endif

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index 94560ba..17f9fe2 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c

@@ -50,6 +50,12 @@
  #  define __NR_bpf 351
  # elif defined(__arc__)
  #  define __NR_bpf 280
+# elif defined(__mips__) && defined(_ABIO32)
+#  define __NR_bpf 4355
+# elif defined(__mips__) && defined(_ABIN32)
+#  define __NR_bpf 6319
+# elif defined(__mips__) && defined(_ABI64)
+#  define __NR_bpf 5315
  # else
  #  error __NR_bpf not defined. libbpf does not support your arch.
  # endif

diff --git a/tools/lib/bpf/skel_internal.h b/tools/lib/bpf/skel_internal.h
index 9cf6670..064da66 100644
--- a/tools/lib/bpf/skel_internal.h
+++ b/tools/lib/bpf/skel_internal.h

@@ -7,6 +7,16 @@
  #include <sys/syscall.h>
  #include <sys/mman.h>
  
+#ifndef __NR_bpf
+# if defined(__mips__) && defined(_ABIO32)
+#  define __NR_bpf 4355
+# elif defined(__mips__) && defined(_ABIN32)
+#  define __NR_bpf 6319
+# elif defined(__mips__) && defined(_ABI64)
+#  define __NR_bpf 5315
+# endif
+#endif